R.K.
Sadly, this is so normal and typical of this age. lol
Watch what's going on to know the full dynamic. Talk to both children; if he's biting because he's feeling picked on or "defending" himself, sister needs to be attended to as well.
I've always grabbed my biters and tapped their mouths (tap=one flat finger between the fist and second knuckle)one or two times and tell them "NO SIR" or "NO MA'AM", hold him/her while comforting the bitten one and tell my biter "owie" then I take them to the kitchen and give them something that IS for biting, "brother/sister isn't for biting, THIS is for biting" I also talk to my bitten one and walk through the scenario, "if you see him/her coming at you, MOVE...if you're being mean that's the only thing s/he has to get you back with so BE NICE!" Biting usually only happens a couple of times with my little ones.